Air bags are required to be capable of fully protecting an occupant from a shock when an accident occurs.
To meet this requirement, a woven fabric for producing the air bag is required to have the following performances.
(1) In the initial opening and expanding (inflating) stage of the air bag made from the woven fabric, no local stress is created on the air bag.
(2) In a final inflated form of the air bag, the amount of energy capable of being absorbed by the air bag when an crash occurs is large.
(3) In the final inflated form, the air bag exhibit a uniform air permeability and the largest resistance to inner pressure of the air bag when it is inflated is constant.
A typical woven fabric for the conventional air bags is disclosed in Canadian Patent No. 974745. This woven fabric is produced by heat-shrinking a gray nylon woven fabric by using a tenter under no tension and thus exhibits a tensile strength of 400 pounds/2.5 cm (about 181 kg/2.5 cm) or more measured by a grab method both in the warp and weft directions of the woven fabric and a low air permeability of 160 cfm or less measured under a pressure of 5 pounds/inch.sup.2 (about 7.1 liters/dm.sup.2 /min or less under a pressure of about 500 Pa).
Also, Japanese Unexamined Patent Publication (Kokai) No. 3-139245 discloses a non-coated nylon fabric for air bags. This woven fabric has been produced by scouring a gray woven fabric and heat-treating the second woven fabric, and thus exhibits a tensile strength of 2,900 N/5 cm (about 178 kg/3 cm) both in the warp and weft directions of the woven fabric and a low air permeability of 10 liters/dm.sup.2 /min under a pressure of 500 Pa.
The above-mentioned prior art publications does not mention the above-mentioned requirements (1), (2) and (3) for the air bag woven fabric and therefore does not teach or suggest means for satisfying the requirements.
Further, Japanese Unexamined Patent Publication (Kokai) No. 4-214,437 discloses a polyester woven fabric for air bags. The woven fabric is not processed through a scouring and a heat treatment and exhibits, even where the basis weight thereof is 200 g/m.sup.2 or less, a tensile strength of 220 daN/5 cm (about 135 kg/3 cm) or more and an ultimate elongation of 25% or more.
Further, U.S. Pat. No. 4,977,016 corresponding to Japanese Unexamined Patent Publication (Kokai) No. 4-2,835 discloses a polyester woven fabric for air bags, having a tensile strength of 1334 N/inch (161 kg/3 cm) or more and an ultimate elongation of 25% or more.
Furthermore, EP-A1-0442,373 specification discloses polyester woven fabrics. The woven fabrics are not processed through scouring and a heat-treatment, and exhibit an ultimate elongation (tensile elongation at break) of 25% or more and an air permeability of 4.7 to 9.4 liter/dm.sup.2 /min under a pressure of 50 mm Aq. However, the ultimate elongations of the woven fabrics in the weft direction thereof are all less than 30% and the EP specification is completely silent as to the uniformity of the fabrics throughtout the fabrics. The woven fabrics in the gray state are subjected to much stress and strain created during the weaving procedure and remaining in the fabrics, and thus are generally not appropriate as woven fabrics for air bags.
Namely, the above-mentioned prior arts contain no mention of the above-mentioned requirements (1) to (3) and thus are quite silent as to a means for satisfying the requirements. Nevertheless, recently air bags having very high burst strength are strongly demanded.
Particularly, for light weight and compact air bags which are recently making rapid progress, the requirement of the high burst strength is assuming greater and greater importance.